This invention relates to laser processing systems and methods, including systems and methods for removing, with high yield, closely-spaced metal link structures or “fuses” on a silicon substrate of an integrated circuit or memory device.
Laser systems can be employed to remove fuse structures (“blow links”) in integrated circuits and memory devices such as ASICs, DRAMs, and SRAMs, for purposes such as removing defective elements and replacing them with redundant elements provided for this purpose (“redundant memory repair”), or programming of logic devices. Link processing laser systems include the M320 and M325 systems manufactured by General Scanning, Inc, which produce laser outputs over a variety of wavelengths, including 1.047 μm, 1.064 μm, and 1.32 μm.
Economic imperatives have led to the development of smaller, more complex, higher-density semiconductor structures. These smaller structures can have the advantage of operation at relatively high speed. Also, because the semiconductor device part can be smaller, a greater number of parts can be included in a single wafer. Because the cost of processing a single wafer in a semiconductor fabrication plant can be almost independent of the number of parts on the wafer, the greater number of parts per wafer can translate into lower cost per part.
In the 1980s, semiconductor device parts often included polysilicon or silicide interconnects. Although poly-based interconnects are relatively poor conductors, they were easily fabricated using processes available at the time, and were well-suited to the wavelengths generated by the Nd:YAG lasers commonly available at the time. As geometries shrank, however, the poor conductivity of polysilicon interconnects and link structures became problematic, and some semiconductor manufacturers switched to aluminum. It was found that certain conventional lasers did not cut the aluminum links as well as they had cut polysilicon links, and in particular that damage to the silicon substrate could occur. This situation could be explained by the fact that the reflection in aluminum is very high and the absorption is low. Therefore, increased energy must be used to overcome this low absorption. The higher energy can tend to damage the substrate when too much energy is used.
Sun et al., U.S. Pat. No. 5,265,114 advances an “absorption contrast” model for selecting an appropriate laser wavelength to cut aluminum and other metals such as nickel, tungsten, and platinum. In particular, this patent describes selecting a wavelength range in which silicon is almost transparent and in which the optical absorption behavior of the metal link material is sufficient for the link to be processed. The patent states that the 1.2 to 2.0 μm wavelength range provides a high absorption contrast between a silicon substrate and high-conductivity link structures, as compared with laser wavelengths of 1.064 μm and 0.532 μm.